Heat spreader

ABSTRACT

A flexible, self-contained active multi-phase heat spreader apparatus for cooling electronic components, the heat spreader having fluid sealed between two plates and a pumping mechanism to actuate multi-phase flow of the fluid. Thermal energy from an electronic component in contact with the heat spreader is dissipated from a core region via the working fluid to the entire heat spreader, and then to a heat sink. Surface enhancement features located between the two plates aid transfer of thermal energy from a first metal plate into the fluid.

FIELD OF THE INVENTION

This invention relates to a heat spreader and particularly relates toactive heat spreaders in contact with microprocessor chips.

TECHNICAL BACKGROUND

In the field of microprocessor chip packages, an effective transfer ofthermal energy from a microprocessor chip to a heat sink is importantfor thermal management of the microprocessor chip.

Generally, the heat sink is of relatively larger area than themicroprocessor chip and can be cooled by forced air conduction. Thethermal energy is transferred to the passive heat sink solely by heatconduction through solid structures. Generally, these structures aremetal and so the thermal resistance of the metal scales inversely withthe cross-sectional area of the conduction path, resulting in atemperature difference between the microprocessor chip and the peripheryof the heat sink which mitigates efficiency. Other materials for theheat sink may be considered, for example, diamond has a relatively lowthermal resistance which would make it a suitable material for a largearea heat sink. However, this is a costly material which is difficult topattern in 3-dimensions; and, an additional thermal interface would berequired for the conduction of thermal energy from the microprocessorchip to the diamond heat sink.

As an alternative to the passive heat spreader technique, impingementcooling and microchannel cooling are also known in the art. Impingementcooling involves liquid flowing through channels that impinge normal tothe back side of a microprocessor chip. Microchannel cooling involvesliquid flowing through parallel channels along the back side of amicroprocessor chip, wherein the liquid is subject to forced convection.Both techniques require an external pump to circulate the liquid betweenthe back side of the microprocessor chip, where heat is absorbed, and aradiator. Heat is subsequently dissipated from the radiator to ambientusing a fan and an air heat exchanger. Disadvantages of these coolingtechniques include the relatively large weight and size of theapparatus, which makes them incompatible with portable systems. The pumpcauses increased noise levels and reduced system reliability. Further,at each interconnect (i.e., between chip, pump and heat exchanger) apressure drop is experienced which reduces overall system efficiency.The necessary fluidic fixtures, pumps, sealing and system assembly arecostly.

Another known cooling device is an active linear heat spreader device,which uses sinusoidally oscillating flow of a working fluid along anetwork of tubes. U.S. Pat. No. 6,631,077 discloses an active heatspreader for microprocessor chip cooling. It comprises a plate with aplurality of interconnected parallel channels, a reservoir at a firstend of the plate and an oscillator at a second end of the plate. A coverseals a working fluid into the channels, reservoir and oscillator, theworking fluid having liquid phase flow or liquid-vapour phase flow. Theheat spreader is thermally coupled to the microprocessor chip via a heattransfer medium.

In U.S. Pat. No. 6,655,450, a heat pipe for cooling electronic elementsis disclosed which utilises forced oscillatory linear flow of a chargedliquid. In the heat pipe body, a closed loop flow path meanders betweena heat absorbing section and a heat radiating section. A mechanism tocause the oscillatory flow is sealed at the connection between the twoends of the heat pipe. This mechanism may be a vibrator, a solenoid or adiaphragm.

These active linear heat spreader designs are known to provide effectivethermal conductivity along the length of the liquid-filled channels dueto a relatively high heat flux between the hot and cold reservoirs.However, the high heat flux occurs only when the flow rate reaches thepeak of oscillation and a poor heat flux occurs when the flow stopsbetween cycles.

Such linear oscillating flow systems utilize “undeveloped” flowprofiles. These flow profiles are characterized by a region at thecenter of a channel that has a nearly uniform flow speed with a viscousboundary layer attached to the channel walls. This plug-like velocityprofile is achieved using an appropriate frequency of oscillations for agiven working fluid and fluidic gap (i.e., channel depth). Theundeveloped flow profile allows the heat contained in the region havingthe plug-like profile to be transported farther along the channel beforethe heat is absorbed radially. Thus, a higher degree of mass transportis created compared to a parabolic-type flow profile. Depending on thethermal diffusivity of the working fluid, an undeveloped thermal profilecan also be induced which further enhances the heat transferred by theplug-like velocity profile. In order to theoretically predict theconditions required for the undeveloped flow profiles, analytic modelsfor viscous laminar flows within circular channel are used. Two keydimensionless parameters based on the velocity and thermal boundarylayers are used to estimate the flow profiles. First, for a thinvelocity boundary layer condition to be met, a Womersley number isdefined:

$\begin{matrix}{{Wo} = {\frac{H}{2} \cdot \sqrt{\frac{\omega \cdot \rho}{\eta}}}} & (1)\end{matrix}$

where η, ρ, ω, and H represent the fluid dynamic viscosity, density,oscillation frequency and channel diameter respectively. When W_(o)>>1,a plug-like velocity profile is expected with a viscous boundary at theside walls. Because heat transfer is also of interest, the temperatureprofile should also be similar to the plug-like velocity profile and isdefined using another dimensionless number, α [5]:

$\begin{matrix}{\alpha = {\frac{H}{2} \cdot \sqrt{\frac{\omega \cdot \rho \cdot C_{p}}{k}}}} & (2)\end{matrix}$

where k and C_(p) represent the fluid thermal conductivity and specificheat capacity respectively. For α>>1, a region of uniform temperatureexists at the channel centerline similar to the condition above for thevelocity profile. In order to achieve the plug-like velocity profile fora working fluid of relatively high thermal conductivity, for exampleliquid metal, and a relatively large fluidic gap, an appropriatefrequency of oscillations would be higher than for a working fluid ofwater or oil. Thus, practical application may be limited. For maximumperformance both W_(o) and a should be substantially greater than 1. Theproblem encountered with such flow velocity profiles is that arelatively large pumping power is required in order to further increaseW_(o) and α. Known linear oscillating flow heat spreaders utilize anefficiency relationship to define optimum heat spreader performancewhich takes into account oscillation frequency and amplitude. Accordingto U. H. Kurzweg in “Enhanced heat conduction in oscillating viscousflows within parallel-plate channels”, J. Fluid Mech. (1985), vol. 156,pp. 291-300, the most efficient operation for linearly oscillating flowbetween parallel plates at a fixed operating frequency, occursapproximately when:

$\begin{matrix}{{{Wo}^{2} \cdot \Pr} = {{{Wo}^{2} \cdot \left( \frac{\eta \cdot C_{p}}{k} \right)} = {\frac{H^{2} \cdot \omega \cdot \rho \cdot C_{p}}{4k} = \pi}}} & (3)\end{matrix}$

where Pr represents the Prandt1 number. It is also shown theoreticallythat for an oscillating flow system operating at maximum efficiency, theheat flux between hot and cool reservoirs, with a tidal displacement S,scales as:

q˜ΔTωS ²  (4)

where q and ΔT represent heat flux and reservoir temperature difference,respectively.

A problem associated with conventional microprocessor chip coolingtechniques is a result of the warpage in a microprocessor chip whenheated. This warpage causes the gap between a microprocessor chip and aheat sink to enlarge, thus resulting in a significantly increasedthermal resistance to ambient. It is common in the fabrication ofmicroprocessor chip packages to use thick rigid plates or lids, whichfurther compounds this problem and also adds significant mass to apackage. Additionally, as the warpage of a microprocessor chip changesduring hot and cold states, any thermal interface material between themicroprocessor chip and heat sink is subject to a pumping action, in aneffect known as “paste pumping”. Over many operating cycles, this effectgradually degrades the thermal interface material and hence thereliability of the interface.

It is an aim of the present invention to provide a heat spreader whichmitigates the problems of the known art.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided aheat spreader comprising a chamber comprising first surface, a secondsurface, and chamber sidewalls and a fluid sealed in the chamber,characterized in that at least two actuators are disposed in the chamberand the operational phase of the actuators results in a substantiallyconstant velocity of fluid flow across a core region of the heatspreader. Advantageously, the heat spreader is a self-contained coolingdevice which may directly interface with a microprocessor chip, and theheat spreader cross-section is relatively thin enabling the heatspreader to deform to accommodate microprocessor chip warpage.

Preferably, the direction of the fluid flow changes radially and the atleast two actuators are one of four actuators which drive a two-phaseflow cycle, six actuators which drive a three-phase flow cycle, andmultiple actuators which drive a respective multi-phase flow cycle.Advantageously, the heat spreader has an effective thermal conductivityseveral times higher than diamond.

Each actuator preferably comprises a reservoir of the fluid and amembrane interfacing between an actuation means and the reservoir, andmay additionally comprise a further membrane positioned on the oppositeside of the reservoir from the actuation means. Thus, quiet andefficient operation is provided, and external pumps are not required.

The core region is preferably positioned centrally within the heatspreader and the actuators are positioned at the periphery of the heatspreader.

Surface enhancement features may be positioned on at least one of thefirst surface and the second surface, and may comprise a plurality ofprotrusions in a distributed arrangement, whereby the shape of theprotrusions and the distributed arrangement provide a substantiallyuniform flow resistance along all radial flow directions. Preferably,the distributed arrangement is located at the core region. Further, thesurface enhancement features may comprise at least one flow directingfeature having a form of a barrier positioned to provide substantiallyuniform flow of the fluid throughout the heat spreader, and the at leastone flow directing feature may be located in an area between the coreregion and the actuators.

The fluid may be one of water, oil, a magnetic liquid and a conductiveliquid.

According to a second aspect of the present invention there is provideda heat shuttle comprising a head portion fixed to a surface via a mobilestem portion, where by the heat shuttle transfers thermal energy betweenthe surface and a fluid circulating around the head portion. In apreferred embodiment, the heat shuttle is a surface enhancement featurein a heat spreader of the present concept. Advantageously, thermalconductivity between the first and second surfaces of the heat spreaderand the fluid is enhanced because the effective surface area from whichheat is transferred into the fluid is enlarged by the heat shuttle.

According to a third aspect of the present invention there is provided amethod for transferring heat away from a heat source, the methodcomprising the steps of providing a chamber, comprising first surface, asecond surface and chamber walls, and a fluid sealed in the chamberbetween the first and second surfaces, and at least two actuatorsintegral to the arrangement, and having the chamber in contact with theheat source, an actuating step for actuating the actuators, such thatthe operational phase of the actuators results in a substantiallyconstant velocity of fluid flow across a core region of the arrangement.

The use of the term ‘phase’ in the following description refers to afraction of a cycle of a periodic waveform which has been completed at aspecific reference time, and not to the state of a substance, namelysolid, liquid or gas.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way ofexample only, with reference to the accompanying drawings in which:

FIG. 1 depicts an enlarged schematic plan view of an embodiment of anactive multi-phase heat spreader of the present invention;

FIG. 2 shows a cross-sectional view of an embodiment of an activemulti-phase heat spreader in contact with a heat sink and amicroprocessor chip;

FIG. 3 a shows a schematic illustration of a two-phase actuated flowcycle;

FIG. 3 b shows a schematic illustration of three-phase actuated flowcycle;

FIG. 4 indicates working fluid movement during part of a two-phaseactuated flow cycle;

FIG. 5 shows a plan view of a section of spreading area and reservoirsof an active multi-phase heat spreader showing flow directing features;

FIG. 6 illustrates three alternative arrangements of surface enhancementfeatures within the core region;

FIG. 7 a depicts a side view of a first embodiment of a heat shuttleduring three stages of a flow cycle;

FIG. 7 b depicts a side view of a second embodiment of a heat shuttleduring three stages of a flow cycle;

FIG. 8 shows a plan view of an exemplary embodiment of a series ofpatterned plate layers;

FIG. 9 a shows a cross-sectional view of a preferred embodiment of anactive multi-phase heat spreader;

FIG. 9 b shows a cross-sectional view of a further preferred embodimentof an active multi-phase heat spreader;

FIG. 10 is a graphical illustration showing drive frequency in Hertzagainst the fluidic gap in meters with respect to liquid metal, waterand oil as working fluids; and

FIG. 11 depicts a schematic illustration of symmetric membrane actuationin the fluidic gap of a reservoir.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a plan view of an active multi-phase heat spreader 10, andspecifically an active two-phase heat spreader. A square spreading area12 has a rectangular reservoir 14 at each of the four peripheral edges.A square core region 16 is located at the centre of the heat spreader 10and has approximately the same dimensions as a microprocessor chip whichis positioned below the heat spreader 10. For example, where the coreregion 16 has an area of approximately 1 cm×1 cm, a spreading area 12 ofapproximately 15 cm×15 cm may be utilized. Four mounting holes 18 areprovided at the corners of the core region 16, and a flow directingfeature 20 joins each mounting hole 18 to the nearest spreading areacorner.

FIG. 2 shows a cross-sectional view of an embodiment of the activemulti-phase heat spreader 10 in contact with a large area heat sink 22and a microprocessor chip 34. The microprocessor chip 34 is mounted on amicroprocessor chip carrier 24 which is further attached to a printedcircuit board 26. The microprocessor chip 34 is positioned approximatelybelow the core region 16 of the active multi-phase heat spreader 10.Several actuators 28, also referred to as actuation means, are utilized,each positioned in contact with a membrane cover 30 over a reservoir 14,of which two are seen in this cross-sectional view. Four spring-loadedplungers 32, of which two are seen in this view, pass through theprinted circuit board 26 and through the mounting holes 18 in the activemulti-phase heat spreader 10 and act to hold the components inmechanically biased contact. An area of thermal interface material 35 islocated between the uppermost surface of the microprocessor chip 34 andthe adjacent surface of the heat spreader 10. The heat spreader 10 isgenerally fabricated from layers of metal and has a relatively thintotal depth, which results in an elastic quality. A gap, known as afluidic gap 36, is provided between a first layer of the heat spreader10, also referred to as a first surface 21, and a second layer of theheat spreader 10, also referred to as a second surface 23, and a workingfluid 38 is sealed into the fluidic gap 36. In preferred embodiments, afluidic gap may be in the range of approximately 0.01 mm-1.0 mm. Theworking fluid 38 may be, for example, water, oil or a magnetic orconductive liquid.

In operation, a substantial proportion of heat generated by themicroprocessor chip 34 is conducted via the thermal interface material35 into the working fluid 38 in the fluidic gap 36 at the core region 16of the heat spreader 10. The working fluid 38 is driven through thefluidic gap 36 by a flow created by the venting and filling of thereservoirs 14, where each reservoir operation is initiated by thecontacting actuator 28 moving the membrane 30. The resultant two-phaseactuated, radially oscillating flow pattern causes a constant flow speedacross the core region 16 and distributes heat absorbed by the workingfluid 38 in a radial direction and across the entire area of the heatspreader 10. Also, boiling-enhanced cooling may be involved in thedistribution of heat from the core region 16. Specifically, vapor formsin the fluidic gap 36 at the core region 16 and is distributed via theoscillating working fluid motion.

The primary heat path from the heat spreader 10 to ambient is primarilyvia the entire uppermost plate of the heat spreader, which generallyforms the base of a conventional heat sink. Thus, heat is transferredfrom the core region 16, the spreading area 12 and the reservoirs 14,into the conventional heat sink. For example, where heat density at thecore region is approximately 300 Watts/cm², then heat density will beapproximately 1.3 Watts/cm² for the entire uppermost plate of the heatspreader. This heat density can be dissipated via forced air cooling.

Specifically, the two-phase actuated, radially oscillating flow patternoccurs in the following way, described with reference to FIG. 3 a. Thefour reservoirs notated A, A′, B, B′ are driven by the actuators at 90degrees out of phase. Specifically, as flow in the A-A′ reservoirsbegins to decay to zero, the flow between the B-B′ reservoirs isreaching a maximum, and the flow rotates radially. Thus, the flowdirection rotates 360 degrees during one complete cycle. Advantageously,the constant flow speed across the core region of the active two-phasesystem is at the maximum value of the active single-phase system.

FIG. 3 b schematically shows a plan view of an active three-phase heatspreader of the present concept, wherein six rectangular reservoirs arelocated at the peripheral edges of a hexagonal spreading area. Thethree-phase actuated, radially oscillating flow pattern occurs in asimilar fashion to the two-phase actuated radially oscillating flowpattern. Namely, the six reservoirs notated A, A′, B, B′, C, C′ aredriven by the actuators at 60 degrees out of phase. Specifically, asflow in the A-A′ reservoirs begins to decay to zero, the flow betweenthe B-B′ reservoirs is reaching a maximum, and then as flow in the B-B′reservoirs begins to decay to zero, the flow between the C-C′ reservoirsis reaching a maximum. Again, the flow rotates radially as indicated bythe solid arrows on FIG. 3 b.

FIG. 4 illustrates the circulation of the working fluid within the heatspreading area during two of the intermediate phases of an activetwo-phase system.

Advantageously, active three-phase and higher multi-phase heat spreadersprovide heat spreading of greater uniformity than an active two-phaseheat spreader.

Further measures to provide efficient heat spreading include theaddition of flow directing features located in the spreading area 12 andalso surface enhancement features located in the core region 16.

FIG. 5 depicts a plan view of a portion of an active two-phase heatspreader 10 showing flow directing features 40. Generally, these arebarriers of varying lengths attached between a lower plate and an upperplate in the fluidic gap 36 of the heat spreader 10. Various designs arepossible dependent upon the shape of the spreading area 12 and whetherthe flow directing features 40 have a mechanical secondary role inmaintaining a uniform fluidic gap 36.

In operation, the flow directing features 40 aid the uniform flushing ofthe spreading area 12 with working fluid 38 and also the elimination offlow stagnation zones.

FIG. 6 shows three alternative arrangements of surface enhancementfeatures 42 within the core region 16. The illustrated arrays showpreferred embodiments, namely; a square grid layout, a concentric circlelayout, and a diamond grid layout. The distributed arrays of surfaceenhancement features 42 illustrated are micro-machined posts, fabricatedfrom a highly conductive material. The shape, size and pitch of thesurface enhancement features should be predetermined to provide auniform flow resistance along all radial flow directions. The surfaceenhancement features 42 reduce the thermal resistance between theworking fluid and the inner surfaces of the upper and lower plates byincreasing the effective surface area for heat transfer.

A heat shuttle is a type of surface enhancement feature which oscillatesin position to dissipate heat into the working fluid 38. FIG. 7 adepicts a side view of a first embodiment of a heat shuttle 44. The heatshuttle comprises a stem portion 46, attached via a hinged means to aninner surface of a active multi-phase heat spreader 10, and a headportion 48 attached to the terminal end of the stem portion 46.Specifically, FIG. 7 a shows the heat shuttle position during threestages of a flow cycle. FIG. 7 a.i) illustrates the heat shuttleposition during cross flow, when the head portion 48 is in closeproximity with an inner surface of the lower plate. Heat is loaded intothe heat shuttle 44 from the inner surface. FIG. 7 a.ii) illustrates theheat shuttle 44 position during parallel flow, when the stem portion 46is normal to the inner surface of the lower plate. Heat is dissipatedinto the working fluid. FIG. 7 a.iii) illustrates the heat shuttleposition during cross flow in an opposite direction, and again the headportion 48 is in close proximity with an inner surface of the lowerplate. Thus the heat shuttle oscillates, also known as shuttling, as thefluid flow in the fluidic gap 36 oscillates radially. In an activemulti-phase heat spreader 10, the working fluid flow cycle illustratedin FIGS. 7 a.i)-iii) can be denoted θ=3Π/2, θ=0 or Π, θ=Π/2,respectively.

FIGS. 7 b.i)-iii) illustrate an alternative heat shuttle structure 50.The embodiment illustrated shows a head portion with a trapezoidal crosssection which is attached to a flexible stem portion (not illustrated).Forces created as the working fluid flows across the structure atcertain angles during a flow cycle cause the head portion to be lifted,see FIG. 7 b.i) and iii), and lowered, see FIG. 7 b.ii). The flexiblestem is attached to an inner surface of the heat spreader 10, but mayflex to enable the raising and lowering of the head portion.

Preferably a thermal time constant of the heat shuttle structure, 44 or50, is designed to correspond with the shuttling frequency. More designfreedom in the thermal time constant of the heat shuttle structure, 44or 50, can be achieved by providing a heat shuttle that shifts positionat relatively smaller working fluid flow cycle stages, such as Π/3, Π/4etc.

Various designs of heat shuttle can be envisaged comprising a mass thatshuttles as the working fluid flow rotates through predetermined angles.

The heat shuttles, 44 or 50, may be positioned in an array in the coreregion 16 in a similar manner to the posts 42 described above. Sucharrays should not lead to a preferred flow direction in any particularradial angle. Rather, the heat shuttles, 44 or 50, should provide auniform flow resistance in all radial directions.

FIG. 8 shows a plan view of an exemplary embodiment of a series ofpatterned plate layers utilized in compiling an active two-phase heatspreader 10. Various planar fabrication techniques, such as microscalebatch fabrication could be utilized to create the layers, 52, 54, 56,which are then stacked or bonded together. Polymer membranes 30 may beattached over both sides of the reservoir openings as illustrated inFIG. 9 a. FIG. 9 b illustrates an alternative layered active multi-phaseheat spreader design involving a third layer of a non-patterned plate 58and attaching polymer membranes 30 only over the reservoir opening onthe first layer of a patterned plate 54. Standoffs 60, which arepositioned between the carrier and active multi-phase heat spreader andfunction to support and separate these entities, are also illustrated.

In the present active multi-phase heat spreader concept, the relativelythin upper and lower plates, the distributed surface enhancementfeatures in the core region, and the arrangement of spring loadedplungers in the mounting holes, combine to allow the heat spreader todeform to match the warpage of a cyclically-heated microprocessor chip.Advantageously, as the active multi-phase heat spreader deforms to matchthe warpage of a heated microprocessor chip, thermal energy transfer cancontinue and a small gap is maintained for a thermal interface material.Thus, “paste pumping” is reduced and a stress load associated with rigidsolder or adhesive bonds is avoided.

A “deflection distance” is defined as the distance that working fluid ismoved after a single pump action. This deflection distance can becalculated based on the volume change of the reservoir between a ventedstate and a filled state, and the cross sectional area of the fluidicgap. For a pair of oppositely-located reservoirs working together, whereone of the reservoirs is venting and the other is filling, each with amembrane cross-sectional area A_(r) moving a through a distance d_(r)and a corresponding active multi-phase heat spreader length with averagecross-section of A_(s), the fluid displacement length S in the spreaderis:

S=d _(r) A _(r) /A _(s)  (5)

In a preferred embodiment of the active multi-phase heat spreader, thereservoirs and membrane actuation may be designed such that the volumeof working fluid displaced by a pump action is adequate to move workingfluid at least half the distance between oppositely located reservoirs.Thus, the required reservoir deflection for a known spreader length is:

d _(r) =SA _(r) /A _(r)  (6)

In a preferred embodiment of the active multi-phase heat spreader, themembrane actuators may be designed to operate at a resonant frequencyrelated to the elastic stiffness of the membrane 30 and the mass ofworking fluid 38 in oscillation in order to reduce the electrical powerrequired by the pump action means. The resonant frequency preferablyalso corresponds to the operating frequency for maximum heat transferfrom a fluidic heat transfer model such as equation 3.

It is possible to determine a minimum membrane actuation frequency, or‘drive frequency’, for a predetermined fluidic gap dimension andspecific working fluid 38. Such a calculation is based on the previouslyrecited equations characterizing undeveloped flow profiles. However,those equations are based on linear oscillating flow within channels ofuniform cross section. Thus, the equations characterizing undevelopedflow profiles of the present active multi-phase heat spreader would bemarginally different, due to the nozzle-like effect as working fluidpasses through the restricted core region 16 and also the radial natureof the flow. FIG. 10 provides a graphical illustration of thisinformation, showing the drive frequency in Hertz against the fluidicgap in meters for liquid metal, water and oil as working fluids. Forexample, where the working fluid is water sealed within a fluidic gap ofapproximately 0.0001 m, then the minimum drive frequency isapproximately 100 Hertz. For a working fluid of oil sealed within afluidic gap of approximately 0.0005 m, the minimum drive frequency isapproximately 10 Hertz.

In the present active multi-phase heat spreader embodiment, the radiallyoscillating flow pattern is driven by membrane actuation causing a pumpaction. FIG. 11 illustrates a symmetric membrane actuation in thefluidic gap 36 of a reservoir 14, where the left diagram shows thereservoir in a vented state and the right diagram shows the reservoir ina filled state. Such symmetric membrane actuation may reduce vibrationgenerated by the oscillating membranes 30, compared to an actuatorembodiment having a single membrane 30. The membrane actuation may beperformed by a solenoid or electrostatic forces or piezo layers on amembrane surface. Where the working fluid 38 is a conductive or magneticliquid, then a magnetic pump action may be utilized. Also, in a furtherpreferred embodiment, the membrane actuators may be designed to allowfor the application of both push and pull forces, as indicated by thesolid arrows in FIG. 11, to ensure reservoirs are vented and filled withan appropriate phase shift and without cross-talk between reservoirs.

Alternatives to various aspects of the above described embodiments of anactive multi-phase heat spreader are envisioned. For example, it is alsoenvisioned that the surface enhancement features can be fabricated onthe inner surface of either or both of the upper and lower plates. Themicroprocessor chip and associated core region can be of various shapesin addition to the square shape of the illustrated embodiments. Whilstthe embodiments described focus on heat spreader plates and reservoircovers comprised of copper and polymer respectively, other suitablematerials may be utilized. In addition to the possible pump action meanspreviously recited, other known pump action means for membrane actuationcan be utilized. Whilst a preferred embodiment of the heat spreader hasa layer of thermal interface material between the microprocessor chipand the lower heat spreader plate, the provision of thermal interfacematerial is optional.

It is envisioned that the above-disclosed heat shuttle concept may beutilized outside the confines of active multi-phase heat spreaders. Forexample, active single-phase heat spreaders have an oscillating flowsimilar to the oscillating flow of the active multi-phase heat spreader,and could therefore also utilize the heat shuttle concept. Further,microchannel cooling devices have a pulsing flow which is also suitablefor heat shuttle implementation. Specifically, as the pulsing flowperiodically reduces and surges, a heat shuttle may absorb and dissipateheat respectively. Further, any scenario requiring transfer of heatenergy from a solid into a fluid, on any scale, may utilize this heatshuttle concept. Where the fluid has neither oscillating nor pulsingflow, then the heat shuttle may be mechanically moved between contactwith the heated surface and the fluid. The scope of the use of the heatshuttles may extend to any scenario where transfer of heat from a solidbody into a fluid is required.

It will be appreciated that the active multi-phase heat spreader of thepresent concept may be interfaced with a plurality of high power densitydevices, for example, high power laser and optics components or powerelectronics. Also, direct integration of the active multi-phase heatspreader to various devices may be realized.

Improvements and modifications can be made to the foregoing withoutdeparting from the scope of the present invention as set forth in theappended claims.

1. A heat spreader comprising: a chamber comprising first surface, asecond surface and chamber sides; fluid sealed in said chamber, and sixactuators disposed within said chamber, whereby an operational phase ofthe actuators in a three-phase flow cycle results in a substantiallyconstant velocity of fluid flow across a core region of the heatspreader, wherein the direction of the fluid flow changes radially. 2.The heat spreader as claimed in claim 1, wherein the core region ispositioned centrally within the chamber and the actuators are positionedat the periphery of the chamber.
 3. The heat spreader as claimed inclaim 1, wherein at least one surface enhancement feature is positionedon at least one of said first surface and said second surface.
 4. Theheat spreader as claimed in claim 3, wherein said at least one surfaceenhancement feature comprise a plurality of protrusions in a distributedarrangement to provide a substantially uniform flow resistance along allradial flow directions.
 5. The heat spreader as claimed in claim 4wherein said distributed arrangement is located at the core region. 6.The heat spreader as claimed in claim 3, wherein said at least onesurface enhancement feature comprise at least one flow directing featurecomprising a barrier positioned to provide substantially uniform flow ofthe fluid throughout the chamber.
 7. The heat spreader as claimed inclaim 6, wherein said at least one flow directing feature is located inan area between the core region and the actuators.
 8. The heat spreaderas claimed in claim 1 wherein the fluid is one of water, oil, a magneticliquid and a conductive liquid.
 9. The heat spreader as claimed in claim3 wherein said at least one surface enhancement feature comprises a heatshuttle comprising a head portion fixed to one of said surfaces via amobile stem portion, wherein said heat shuttle transfers thermal energybetween said surface and fluid circulating around said head portion. 10.A heat spreader comprising: a chamber comprising first surface, a secondsurface and chamber sides; fluid sealed in said chamber, and at leasttwo actuators disposed within said chamber, whereby an operational phaseof the actuators results in a substantially constant velocity of fluidflow across a core region of the heat spreader, wherein each actuatorcomprises a reservoir of said fluid and a membrane interfacing betweenan actuation means and said reservoir and additionally comprises afurther membrane positioned on an opposite side of said reservoir fromsaid actuation means.
 11. The heat spreader as claimed in claim 10,wherein the core region is positioned centrally within the chamber andthe actuators are positioned at the periphery of the chamber.
 12. Theheat spreader as claimed in claim 10, wherein at least one surfaceenhancement feature is positioned on at least one of said first surfaceand said second surface.
 13. The heat spreader as claimed in claim 12,wherein said at least one surface enhancement feature comprise aplurality of protrusions in a distributed arrangement to provide asubstantially uniform flow resistance along all radial flow directions.14. The heat spreader as claimed in claim 13 wherein said distributedarrangement is located at the core region.
 15. The heat spreader asclaimed in claim 12, wherein said at least one surface enhancementfeature comprise at least one flow directing feature comprising abarrier positioned to provide substantially uniform flow of the fluidthroughout the chamber.
 16. The heat spreader as claimed in claim 15,wherein said at least one flow directing feature is located in an areabetween the core region and the actuators.
 17. The heat spreader asclaimed in claim 10 wherein the fluid is one of water, oil, a magneticliquid and a conductive liquid.
 18. The heat spreader as claimed inclaim 12 wherein said at least one surface enhancement feature comprisesa heat shuttle comprising a head portion fixed to one of said surfacesvia a mobile stem portion, wherein said heat shuttle transfers thermalenergy between said surface and fluid circulating around said headportion.